1. Field of the Invention
The present invention relates to a display device, and particularly to a liquid crystal display device which is small in decrease in transmittance and low in pixel defect rate even on a high definition screen.
2. Description of the Related Art
In a liquid crystal display device, liquid crystal is sandwiched between a thin film transistor (TFT) substrate and a counter substrate. The TFT substrate has pixels formed in a matrix thereon and each including a pixel electrode, a TFT and so forth. The counter substrate opposed to the TFT substrate has color filters formed at places thereof corresponding to the pixel electrodes of the TFT substrate. The transmittance of light by liquid crystal molecules is controlled for each pixel to form an image. It is to be noted that a pixel signifies a region surrounded by each of scanning lines extending in a first direction and arrayed in a second direction and each of video signal lines extending in the second direction and arrayed in the first direction.
Since a liquid crystal display device is flat and light in weight, applications thereof are spreading in various fields. In portable telephone sets, digital still cameras (DSCs) and so forth, a liquid crystal display device of a small size is used widely. In a liquid crystal display device, the viewing angle characteristic matters. The viewing angle characteristic is a phenomenon in which the luminance varies or the chromaticity varies depending upon whether a screen image is viewed from the front or from an oblique direction. An in-plane switching (IPS) method wherein liquid crystal molecules are operated by an electric field applied in a horizontal direction is superior in the viewing angle characteristic.
Among various IPS methods, some methods have become the mainstream today because the transmittance can be increased. The increased transmittance can be realized by, for example, forming a common electrode solidly in plane and disposing pixel electrodes having a slit above the common electrode with an insulating film interposed therebetween such that liquid crystal molecules can be rotated by an electric field generated between the pixel electrodes and the common electrode. The common electrode and the interlayer insulating film are formed on an organic passivation film which serves also as a flattening film.
On the other hand, if the size of the pixels on a high definition screen of a liquid crystal display device is reduced, then the proportion of the diameter of through-holes for connecting the pixel electrodes and the source electrodes of the TFTs to each other increases.
If it is tried to decrease also the diameter of the through-holes as the pixel size is decreased to raise the definition of the screen, then the taper angle of a wall portion of the through-holes (such taper angle is hereinafter referred to sometimes as taper angle of a through-hole) must be increased. Meanwhile, although an orientation film is used in order to initially orient the liquid crystal, a material for the orientation film is applied by flexographic printing, ink jet printing or the like while it initially (before the application) remains in a liquid state.
If the taper angle of a through-hole is increased, then when the orientation film material is applied, a phenomenon occurs in which the orientation film material does not advance into the through-hole due to the surface tension thereof. If the phenomenon occurs, then since the state of liquid crystal differs in a region in which the orientation film is formed thicker, some display irregularities occur there due to leak of light therethrough or the like.
JP-2007-322563-A discloses a configuration wherein the height is changed around a large opening on an upper side of a through-hole so that an orientation film may flow readily into the through-hole. The method of changing the height around a large opening on an upper side of a through-hole in the above-specified patent document is configured such that a step provided on a lower layer film is reflected on the circumference of the large opening on the upper side of the through-hole.
JP-2008-275879-A discloses a configuration for solving a problem that photoresist for formation of pixel electrode patterns does not advance into a recessed portion of a through-hole and a pixel electrode is etched in the through-hole, resulting in failure to establish electric connection of the pixel electrode. In particular, JP-2008-275879-A discloses a configuration wherein a step is provided around a large opening on the upper side of a through-hole so as to facilitate advancement of photoresist into the through-hole. Alto it is disclosed that a similar problem occurs also with application of an orientation film material. Also the method of changing the height around a large opening on the upper side of a through-hole in the above-specified patent document is configured such that a step provided on a lower layer film is reflected on a periphery of the large opening on the upper side of the through-hole.
JP-2013-140386-A discloses a configuration wherein, in a liquid crystal display device of the IPS type, an optical orientation process is used for an orientation film to use also an inner wall of a through-hole as a region for formation of an image. To this end, the above-specified patent document discloses a configuration wherein, also on an inner wall of a through-hole, a slit of a pixel electrode is formed.